01
Two charts, and only one of them is about the ground
The capacity chart answers whether the crane can lift the load. The reaction chart answers what the crane does to what it is standing on. Asking the first document the second question is the commonest confusion in this subject.
The capacity chart gives lifting capacity against radius, for a stated configuration: boom length, jib, counterweight, outrigger extension, and whether the lift is over the front, side or rear. It is what the operator works to and what the lift plan quotes a duty percentage from. It says nothing about outrigger loads.
The outrigger reaction chart gives the force at each outrigger, usually against slew angle and radius, for the same configuration. Not every manufacturer publishes one for every machine, and it is often a separate document that has to be asked for. This is the one a ground bearing calculation needs.
Two more things sit alongside them and are worth naming because they get confused with both.
The gross weight breakdown. Carrier, superstructure, counterweight, boom and attachments, with their positions. Some manufacturers publish this instead of a reaction chart, which is enough to make an estimate and not enough to replace the chart.
The tipping-line data. Where the machine's stability limits are. Relevant to whether the crane stays upright and not to what the ground receives.
01What are you trying to find out?
- Can the crane lift it
- The capacity chart, at the governing radius. The governing radius is the worst the operation reaches, not the pick radius.
- What the ground receives
- The outrigger reaction chart, swept through the permitted slew arc. Then the mats, then the ground. The chart is the first of three steps and the only one the manufacturer supplies.
- Whether the crane will tip
- The capacity chart already answers it, within its own configuration. Chart capacities are already limited by stability or structure, whichever governs. A ground calculation is not a stability calculation.
02Do you have the reaction chart?
- Yes
- Use it, sweep it, and record the page and revision.
- No, but the weight breakdown is published
- Estimate the reactions by statics, and say on the output that it is an estimate. In this article's worked case that estimate landed 10 percent below the chart and picked a different worst slew angle.
- Neither
- Ask the crane supplier. This is a normal request and it is their document. Designing on a guessed machine weight distribution is not a calculation anyone can review.
02
Reading a reaction chart without misreading it
Five things to check before using a number off one, and four of them are about whether the row you are reading describes the machine standing in front of you.
Configuration. Outrigger extension is the one that catches people: a chart row for fully extended outriggers does not describe a machine on intermediate extension, and the reaction rises on the shorter base because the same overturning moment is resisted over a narrower footprint. Counterweight, boom length and jib all matter the same way.
Which corner. Reaction charts identify the legs, usually as front and rear, left and right, relative to the carrier rather than to the boom. A number read against the wrong leg is a number for a different lift.
What the slew angle is measured from. Usually from over-the-front, but not always, and a chart that measures from over-the-rear will look identical and be 180 degrees out.
Whether the load is included. Some charts give reactions for the machine alone and expect the hoisted load added; others include it. This produces the largest single error in this subject when it goes wrong.
Whether the values are factored. Some manufacturers publish reactions with an allowance already applied. Applying your own dynamic factor on top then double-counts.
The habit that prevents all five: write the chart's identity into the calculation. Machine, configuration, page, revision, date. Not because it is tidy, but because it makes the calculation re-checkable by someone who was not there.
03
What the chart does not tell you
Everything from the underside of the float downwards. The chart stops where the machine stops, and the calculation you actually need starts there.
A reaction chart hands you a force at a point. Between that force and a defensible ground pressure sit three things it says nothing about:
The float. The chart gives a force, and the float's contact area turns it into a pressure. That pressure, in the worked case below, is several times any soil's capacity, which is why mats exist at all.
The mat. How much of the mat's area the calculation may credit depends on the mat's stiffness, and that is a claim you make and defend. The mat also has its own bending and punching checks, and it can fail those while the ground pressure passes.
The ground. The allowable bearing value at the actual setup position, with its provenance.
None of those is the crane manufacturer's business, and none of them is optional.
Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render
The chart, swept, and everything downstream of it
A 160 t class machine with the manufacturer's reactions declared for five slew angles.
| Worst outrigger loadFL at 90 degrees of slew | 806kN |
|---|---|
| Float contact area0.65 m square | 0.42m2 |
| Pressure the float would put into the groundwhich is why mats exist | 1908kPa |
| Mat30 degree spread declared | 3.0 x 3.0 x 0.6m |
| Area the calculation credits | 1.80m2 |
| Pressure on the ground | 447kPa |
| Ground bearing utilisation | 89.4% |
Note the two pressures. The chart's force under the float alone is over a megapascal; the same force under a 3.0 m mat with its spread declared is a few hundred kilopascals. Every part of that reduction is your calculation, not the manufacturer's.
Open this example in the calculator04
When you have not got the chart
Estimate it honestly, label it as an estimate, and know which direction the error runs. On the worked machine the estimate was 10 percent low, and it picked a different governing angle.
With the machine's masses and their positions you can estimate the outrigger reactions by rigid-body statics on the support rectangle.
The estimate, as statics
Three equations for four reactions, so the fourth comes from an assumption about how the indeterminacy resolves - which is the first thing a chart knows and an estimate does not.
That is a real calculation. It is not a replacement for the chart, for a specific reason: it knows about weights and geometry, and it does not know about the machine. It cannot see how the outrigger beams distribute into the carrier, what the manufacturer's own structural limits impose, or how the machine's stability system behaves near its limits.
Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render
The same machine with the reactions estimated
Identical masses, radii, mats and ground. The only change is that the reactions are computed from the machine's geometry rather than read from the chart.
| Worst outrigger load, estimatedagainst 806 kN from the chart | 724kN |
|---|---|
| Difference from the chartthe estimate is the lower number | -10.1% |
| Governing slew angle, estimatedagainst 90 degrees from the chart | 43.4deg |
| Pressure on the groundagainst 447 kPa | 402kPa |
| Ground bearing utilisationagainst 89.4% | 80.3% |
Two findings, and the second is the more important. The estimate is 10 percent below the chart, which is the wrong direction. And it identified a completely different governing slew angle, so an engineer working from it would have written a different restriction into the lift plan.
Open this example in the calculatorThe estimate found the wrong worst case, not just a wrong number. That is the specific danger, and it is why an estimate is a stopgap rather than an alternative. A calculation that names the wrong governing angle produces a lift plan whose controls are aimed at the wrong part of the operation.
05
Radius, and why it is the sensitive input
Everything on a chart is indexed by radius, and radius is measured to the load rather than to where anyone thinks the load is.
Radius is the horizontal distance from the crane's slew centre to the load. Three things make it larger than the number on the plan:
Boom deflection. A loaded boom deflects, which pushes the head outwards. On a long boom at a large radius this is not a small correction.
Load swing. A load that is not directly under the head has a longer radius than the geometry says.
Setting position error. The crane is set up a metre from where the plan drew it, and the radius follows.
Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render
One metre more radius than assumed
The same estimate with the load radius one metre longer. Nothing else changes.
| Load radiusagainst 16.0 m assumed | 17.0m |
|---|---|
| Worst outrigger loadagainst 724 kN at 16.0 m | 750kN |
| Increase3.5 percent | 25kN |
| Ground bearing utilisation | 83.1% |
Three and a half percent on the worst outrigger for one metre. On the capacity side the same metre is usually worth several times that, because chart capacity falls faster with radius than outrigger load rises, which is why the radius is worth pinning down even when the ground check looks comfortable.
Open this example in the calculator06
What the rules require of the chart
That the crane is operated inside it, and that the ground under it is adequate. They are separate duties on separate parties, and neither includes doing your arithmetic.
Before a chart number enters a calculation
- 01Record the chart's identityMachine, configuration, page and revision, in the calculation, not in an email.
- 02Confirm it is the reaction chartOutrigger forces, not lifting capacities. They are different documents.
- 03Confirm the configuration matchesOutrigger extension especially, then counterweight, boom and jib.
- 04Establish whether the load is includedMachine only, or machine plus hoisted load. Stated explicitly in the calculation.
- 05Establish whether the values are already factoredIf they are, your dynamic factor does not go on top of them.
- 06Sweep the permitted slew arcThe governing angle was found rather than assumed, and it is named in the output.
- 07Use the governing radiusThe worst the operation reaches, with an allowance for boom deflection and setting error.
- 08Label an estimate as an estimateIf no chart was available, the output says so, and the lift plan carries that limitation.
Common questions
- Does a crane load chart give outrigger loads?
- The capacity chart does not; the outrigger reaction chart does, and they are separate documents. The capacity chart gives lifting capacity against radius for a stated configuration and is what the operator works to. The reaction chart gives the force at each outrigger, usually against slew angle and radius, and it is the one a ground bearing calculation needs. Not every manufacturer publishes one for every machine, and it often has to be asked for.
- What should I check before using a number from a reaction chart?
- Five things. That the configuration matches, especially outrigger extension. That you are reading the right corner, since charts identify legs relative to the carrier rather than the boom. What the slew angle is measured from, because a chart referenced to over-the-rear looks identical and is 180 degrees out. Whether the hoisted load is already included. And whether the values are already factored, because your dynamic factor does not go on top of one that is.
- Can I estimate outrigger loads without the chart?
- You can calculate them from the machine's masses and their positions by rigid-body statics, and it is a real calculation, but it is not a substitute. It knows about weights and geometry and it cannot know the machine: how the outrigger beams distribute into the carrier, or what the manufacturer's own structural limits impose. In this article's worked case the estimate came out 10 percent below the chart and identified a completely different governing slew angle, so a lift plan written from it would have restricted the wrong part of the operation.
- Does the load chart tell me the ground bearing pressure?
- No. It stops at the force coming out of the machine. Between that force and a defensible ground pressure sit the float's contact area, the mat and how much of it the calculation may credit, and the ground's allowable value with its provenance. In this article's worked case the force under the float alone is over a megapascal and the same force under a 3.0 m mat with its spread declared is a few hundred kilopascals, and every part of that reduction is your calculation.
- How sensitive are outrigger loads to radius?
- Enough to matter, and radius is larger in practice than on the plan because of boom deflection, load swing and setting position error. In the worked estimate, one metre more radius than assumed added 25 kN to the worst outrigger, about three and a half percent. On the capacity side the same metre is usually worth considerably more, because chart capacity falls faster with radius than outrigger load rises.
Sources
Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.
ASME B30.5Mobile and Locomotive Cranes
ASME · paid document
Construction, installation, operation, inspection and maintenance of mobile cranes in the US, including load rating and the requirement to operate within the manufacturer's chart. It governs the machine; the ground it stands on is 29 CFR 1926.1402 and the calculation is yours.
29 CFR 1926.1402Ground conditions (Cranes and Derricks in Construction)
US Occupational Safety and Health Administration · free to read
Requires ground supporting a crane to be firm, drained and graded sufficiently for the manufacturer's specifications, and places the duty to prepare it, and to disclose voids and buried services, on the controlling entity. It states a duty; it prescribes no calculation.
29 CFR 1926 Subpart CCCranes and Derricks in Construction
US Occupational Safety and Health Administration · free to read
The whole US construction crane subpart, free in full: ground conditions, assembly and disassembly, power line clearance, operator qualification, signals, inspection and multiple-crane lifts. The index page, because the duty a reader needs is usually two sections away from the one they searched for.
LOLER 1998Lifting Operations and Lifting Equipment Regulations
UK Health and Safety Executive · free to read
The UK duty framework for lifting operations: planning by a competent person, supervision, and thorough examination of lifting equipment and accessories. Like OSHA's rules it governs the process, not the arithmetic.
Run the check properly
Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.
Something here wrong, or thinner than it should be? Tell us which paragraph and it gets rewritten. Articles carry the date they were last revised for exactly this reason.